Shale gas is natural gas that is trapped in impermeable shale rock, as opposed to more conventional natural gas deposits that are trapped below a layer of impermeable rock.

Therefore simply drilling down to it is not enough and the rock must also be fractured in order to allow the gas to escape. The gas is held in fractures, pore spaces and adsorbed on to the organic material of shale.

The most common way to extract shale gas is by cracking the rock using hydraulic fracturing method. This is often referred to as ‘fracking’.

Fracturing fluid, which is a combination of millions of gallons of water, sand and potentially toxic chemicals, is pumped at high pressure into the rock to create narrow fractures that allow the gas to flow into the well bore and to the surface.

The potentially toxic cocktail of chemicals used in the process have the capability to contaminate the land and water supply.

Operators should disclose, either on their own website or on third-party websites, the chemicals used in their fracturing fluid.

Once the fractures have been created, small particles, usually sand, are pumped into the fractures to keep them open when the water is taken back up the well.

Carboniferous Age shales and Jurassic Age mudstones in Somerset may potentially contain shale gas.